1. Field of the Invention
This invention relates to an improved method and apparatus for controlling the plating thickness on a metal strip which is passed through an electrochemical plating line.
A typical plating line has a plating bath through which a workpiece to be plated is moved. As the workpiece is moved through the plating bath, it is maintained at a substantially constant electrical potential relative to a number of plating electrodes positioned in the bath. The potential difference is maintained between the plating electrodes and the workpiece by a number of rectifier units.
As the workpiece moves through the bath, electrically charged ions in that bath combine with electrons from the recitifier units to form a plating which coats the workpiece. The number of these combinations of ions and electrons, per unit time, represents the plating current within the plating bath. The total plating current in the plating line is equal to the sum of the plating currents from the individual electrodes or rectifier units.
The magnitude of the plating current is dependent upon a number of system variables. It is known within the art, that the input voltage to the rectifiers, the spacing between the electrodes and the workpieces, and the physical condition of the electrodes, all affect the magnitude of the plating current.
In one typical operation, the workpiece is a strip of steel which is passed through the bath in order that a galvanizing coating of zinc may be deposited on its surface. A coil of un plated strip is unwound, plated in the bath, and then rewound as a plated coil. The details of a typical plating arrangement whose efficiency can be improved by the present invention can be found in U.S. Pat. No. 3,468,783 to Avellone which has been assigned to Republic Steel Corporation and which is incorporated by reference here.
2. Description of the Prior Art
For a strip such as that described in the Avellone patent, and for a given length of travel through the bath, the plating thickness is a function of the plating current, the speed with which the strip moves through the bath, and the physical dimensions of the strip. Since it is the plating thickness that an operator is interested in controlling, the above three factors must be taken into account when setting up plating operations. If, for example, the plating current is the only physical parameter controlled, unintended changes in line speed as the strip moves through the bath will affect the plating thickness in an adverse manner. If the plating current is kept constant and the physical dimensions of the object or workpiece are changed, plating thickness will also change. For these reasons, optimum plating thickness can only be achieved through a close monitoring of not only the plating current but also of the speed of the object through the bath and the physical dimensions of the object.
If the physical dimensions of the object or workpiece to be plated and the speed with which the object or workpiece through the media are known, the proper total plating current for a given plating thickness can be calculated. In the past, these calculations were performed for different combinations of the plating material dimensions and plating line speed, and charts expressing the results of the calculation were provided to inform the operator of the correct plating current.
The use of a chart or tabulation often occasioned errors by the operator when calculating the proper plating current for the optimum plating thickness. Sometimes, an operator, in an effort to increase plating production, would increase the speed with which the workpiece moved through the medium without making the needed adjustments in the plating current.
Even if the operator made accurate adjustments for increased plating current, these adjustments could cause the plating rectifiers to exceed their rated current capability. As the operator attempted to increase the speed with which the product was plated (and thereby increase his production), the plating current was also increased and at some point the capacity of the rectifiers was exceeded. When the capacity of a rectifier was exceeded either a circuit breaker was actuated with resultant reduction in production capacity, or even worse, damage to the rectifier occurred producing a reduction in capacity for a longer period of time.
As noted previously, the plating current can also be adjusted by changing the spacing between the electrodes and the workpiece. As an operator attempted to increase the plating current and therefore his production, he would sometimes reposition the electrodes to a position in closer proximity to the object to be plated. Such readjustment sometimes inadvertently brought the electrode in contact with the plating object, causing a short circuit. The resultant high current levels could damage the connected rectifier unit, with the result that total production decreased rather than increased.
When the plating line included a number of electrodes, it was sometimes possible for the prior user to satisfactorily control total plating current, but he had no way of knowing whether this plating current was being efficiently distributed among the large number of rectifier units on the line. Typically, rectifier current levels vary among the units, due for example to nonuniformity in condition of electrodes. Some operate at or near capacity, while others operate at much lower current. Those operating at high levels carry most of the load while the rectifiers at lower current levels operate inefficiently.
A technique for controlling individual plating current in a plating rectifier has been proposed. The thrust of the proposal, is toward maintaining the rectifiers within safe operating parameters, with no regard to apportionment of plating currents. Specifically, each rectifier unit is current limited to maintain the rectifier operating temperature below a maximum safe operating level. The speed of strip movement is apparently controlled as a function of total plating current while operating parameters are monitored. The proposal does not however, describe the manner of the control.